Crosslinked heat recoverable thermoplastic polyurethanes

ABSTRACT

This disclosure describes a class of thermoplastic polyurethane rubbers having heat-activated dimensional memory characteristics. Cross-linking of thermoplastic urethanes is carried out both by chemical means and by high-energy radiation.

limited States Patent [72] Inventor Edward C. Stivers Atherton, Calif. [2]] Appl, No. 502,623 [22] Filed Oct. 22, I965 [45] Patented Nov. 30, 1971 [73] Assignee Raychem Corporation Redwood City, Calif.

[54] CROSSLINKED HEAT RECOVERABLE THERMOPLASTIC POLYURETHANES 5 Claims, No Drawings [52] U.S. Cl 260/77.5, 204/l59.19, 204/1592, 260/75 [51] Int. Cl ..C08g 22/04, C08g 53/20, BOlj 1/10 [50] Field of Search 260/75 T, 75 NP, 77.5 AB, 75 NO, 75 TN, 77.5 A, 75 NA, 77.5 AX; 204/159.l9, 159.2

[56] References Cited UNITED STATES PATENTS 2,806,835 9/1957 Nischk et al. 260/45.4 2,999,851 9/1961 Elmer 260/75 3,036,042 5/1962 Schmidt et al. 260/75 3.056.l7l l0/l962 Fite l8/59 3,06l,530 l0/l962 Gonsalves 204/154 3,098,832 7/1963 Pooley et al.. 260/2.5 3,250,840 5/ l 966 Procopis 264/ l 75 FOREIGN PATENTS 820,004 9/1959 Great Britaim ..260/77.5 AB UX OTHER REFERENCES Gruber and Keplinger: ind. & Eng. Chem., Vol. 5i, No. 2. February i959 Harrington: Rubber Age, Vol. 82, No.3, Dec. 1957 Saunders and Frisch: Polyurethanes, Part II, pages 377- 384 (lnterscience, New York) 1964 Call No. TP986P6S3 Primary Examiner Donald E. Czaja Assistant E.raminerH. S. Cockeram Attorney-Lyon & Lyon ABSTRACT: This disclosure describes a class of thermoplastic polyurethane rubbers having heat-activated dimensional memory characteristics. Cross-linking of thermoplastic urethanes is carried out both by chemical means and by highenergy radiation.

CROSSLINKED HEAT RECOVERABLE THERMOPLASTIC POLYURETHANE This invention relates to a novel class of heat-recoverable cross-linked polyurethane rubbers having heat-activated dimensional memory characteristics.

Polyurethane elastomers or rubbers are known materials for use in conveyor belts, tubing, and the like. These known polyurethane rubbers are cross-linked and are comparable in many ways to vulcanized natural rubber and possess thephysical property of elastic extensibility. Hence, such polyurethane rubbers will stretch an amount directly proportional to the applied force and will recover upon the removal of the-force. Thus, when the force is removed, the polyurethane rubber returns to its original cross-linked size and shape.

More recently there has been marketed a class of uncrosslinked thermoplastic polyurethane rubbers which behave at ordinary, e.g., room temperature as if they are cross-linked, but which behave as a thermoplastic, i.e., melt and flow', at elevated temperatures. Typical of such thermoplastic polyurethane rubbers is one having the following structural formula: These polymers are sold under the trade name Estane.

Another class of such thermoplastic polyurethane rubber is called Texin These isocyanate-terminated polymers are somewhat higher in molecular weight than Estane" the polymer shown above. The class of polymer sold are prepared from hydroxyl-terminated polyesters, methylene-p-phenylene diisocyanate, and a diol resulting in solid, thermoplastic products.

The classical rubberlike behavior of these uncross-linked polymers at moderate temperatures can probably be ascribed to interchain hydrogen bonding which behave as secondary cross-links, e.g:

Hydrogen Bonding Secondary Crosslinks Polyurethanes of this type are well known and are described in High Polymers, Vol. XVl, Saunders and Frisch, Polyurethanes: Chemistry and Technology, ll. Technology, Part II, lnterscience Publishers, Div. of John Wiley & Sons, New York (1964).

Dimensionally heat-unstable or heat-recoverable thermoplastic polyurethane articles of little utility may be produced by imparting a considerable amount of force or stress to the heated material after initial fabrication, followed by a cold temperature quench to hold the molecules .in-the stressed, usually elongated condition. Subsequently, upon carefully heating, the fabricated product will tend to recover or reform to the original configuration. However, these polyurethanes have essentially no strength at elevated temperatures and thus can be readily deformed to an undesired shape or form. Generally, the hydrogen bonds in these materials disappear with increasing temperature in a manner which may be estimated by the Arrhenius equation ./V Ae,,/RT

where v,/V is the secondary cross-link concentration, A a constant, R the general gas constant, T the absolute-temperature, and E is the activation energy of bonding. Thus at temperature of the order of l20-l60 C. these polyurethanes show a decrease in elastic modulus from a room temperature value of l,800p.s.i. to 5p.s.i.

A primary contribution of the present invention is the provision of articles prepared from thermoplastic polyurethane rubbers which are heat-recoverable and exhibit the property of elastic deformation of extensibility under stress over a wide range of temperatures, i.e., have strength at elevated temperatures.

Accordingly, it is an object of this invention to provide articles prepared from thermoplastic polyurethane rubber capable of changing size and/or shape upon the application of heat and which are useful over a wide range of temperatures.

A further object of the present invention is to provide novel articles prepared from thermoplastic polyurethane rubbers which have been cross-linked, which articles possess dimensional memory characteristics and are capable of returning to an original cross-linked shape upon the application of heat.

Yet another object of this invention if to provide a novel method for the production of heat recoverable articles from polyurethane rubbers which have strength at elevated temperatures.

These an other objects of the present invention will be apparent from the detailed description which follows.

According to the present invention, there is provided a class of thermoplastic polyurethane rubbers which have been crosslinked having over a wide temperature range the property of elastic deformation typical of rubbers which can be made into articles which are capable of changing shape and/or size merely upon the application of heat and recovering to the original or cross-linked shape and size.

As will be more fully explained below, an essential aspect of the present invention is that the thermoplastic polyurethanes of the present invention be cross-linked. As used in the specification, the term cross-linked is intended to mean a material having chemical covalent bonds between polymer chains. Since the cross-1inked polyurethane rubbers of the present invention will, at temperature below about 160 C., contain not only covalent interchain bonding, but also hydrogen interchain bonding, the material of this invention will sometimes be hereinafter referred to as having dual" cross-linking. These compositions will also be referred to herein as crosslinked thermoplastic polyurethanes, i.e., thermoplastic polyurethanes which inherently contain hydrogen bonds which have also been cross-linked by means of additional chemical species. Cross-linking may be accomplished by any suitable method, e.g., irradiation or chemical treatment.

The heat recoverable articles of the present invention are produced by deforming articles comprising cross-linked thermoplastic polyurethanes under conditions such that the articles is substantially permanently deformed and retains a deformed configuration until heated to the recovery temperature of the article. This deformation may be most conveniently carried out while the article is maintained at an elevated temperature, e.g., between about C., and C., depending upon the particular polyurethane used, and then quenching the article at a relatively low-temperature, e.g., 25C. However, such deformation at elevated temperatures is preferred because, among other things, a lesser degree of deformation is required to produce a given degree of permanent deformation. For'example, when identical samples produced according to the present invention were stretched at room temperature and at 100 C., it was found that the former sample required an expansion of 560percent to produce a retained expansion of 100 percent whereas the latter sample required an expansion of about l45percent to produce a retained expansion of lpercent. It is, of course, to be understood that the optimum temperature for deformation will vary from composition to composition and will depend upon the composition of the thermoplastic polyurethane, the degree of cross-linking, etc.

Typical heat-recoverable methods by which polyurethane rubber articles of the present invention may be produced include forming or fabricating into the desired configuration, an article comprising 1. A thermoplastic polyurethane of the type having the following general fonnula:

and R is which has been cross-linked by (a) irradiating such a polymer into which has been incorporated a cross-linking monomer such as N,N-methylene-bis-acrylamide (M- BA), and/or N ,N'-hexamethylene-bis-maleimide, (b) chemically cross-linked, e.g. with an organic peroxide, alone or together with a cross-linking monomer such as those mentioned above.

2.Heating the material to an elevated temperature, e.g., about 100 C., applying an external force to deform the article to the desired heat-recoverable configuration and then quenching the article at a lower temperature while in the deformed state.

After release of the external force, the article remains in a deformed configuration, not returning to its original crosslinked configuration, as would be the case in ordinary rubber. in this state, the article has the property of heat-recoverability. As used herein, heat-recoverability" means that an article is dimensionally heat unstable and may be caused to assume a predetermined configuration and a heat stable condition upon the application of heat alone. In addition to having the property of heat-recoverability, these articles have strength at elevated temperatures and will no melt and flow in the manner of conventional uneross-linked thermoplastic polyurethanes With reference to the specific details involved in the preparation of the cross-linked thermoplastic polyurethane rubbers of the present invention, the following preferred and/or optional procedures should be mentioned. The thermoplastic polyurethane of the above formula may be mixed with a cross-linking monomer and/or chemical cross-linking agent, e.g., peroxide, preferably on a mill of the conventional type. If a peroxide or other cross-linking agent is added, the polyurethane is, of course, not irradiated. Many other conventional polyurethane additives may be added. Thus, carbon black may be added as a filler. Also, antimony trioxide and polyvinyl chloride may be added as a flame retarding system, and carbodiimides added for increased hydorlytic stability. Such other flame retardants such as those disclosed in my copending applications, now U.S. Pat. Nos. 3,340,226 and 3,354,19l may also be added. Such compositions may then be fabricated into articles of any desired shape, and thereafter cross-linked. The cross-linking may be brought about by exposure of the article to high-eneragy radiation, such as that from accelerated electrons, X-rays, gamma ray, alpha particles, beta particles, neutrons, etc. A cross-linking monomer is added to the composition and generally the minimum radiation dosage is on the order of 1 megarad. Alternatively, the cross-linking may be brought about by using chemical means, e.g., 2,5-diemthylhexane-2,5-di-t-butyl diperoxide, which after blending with the thermoplastic polyurethane, may be caused to induce cross-linking by heating the blended mixture under pressure to a temperature of C. for 10 hours or 170 C. for l5 minutes, etc. In general, the organic peroxide or other cross-linking agent is employed in an amount from about 'rto about four parts by weight based on the weight of thermoplastic polyurethane.

The cross-linked thermoplastic polyurethane article is then subjected to stress, preferably while at elevated temperature. This elevated temperature is generally above 100 C. and preferably within the range from about 100 C. to about C. The deformed material is then quenched, and preferably the material is maintained under tension during quenching.

The preferred thermoplastic polyurethanes rubber starting materials for use in this invention are sold under the trade names of Estane and Texin and are described chemically above. These materials are supplied in several different forms as indicated in the following table:

Various tests were performed on the heat-recoverable dual cross-linked polyurethanes of the present invention. The modulus of elasticity at l60-200 C. was obtained by calculation from plots of stress (p.s.i.) (l +Elongation) vs. Elongation. The ultimate strengths were calculated using the initial cross-sectional area. The gel contents of the polymers were determined by extracting the cross-linked systems with boiling tetrahydrofuran following the procedures outlined by Lyons, B. .l., [J. Polymer Sc., Pt. A, 3,p. 777( 1965)]. The procedure is basically a modification of the Charlesby-pinner technique [A. Charlesby and S. H. Pinner, Proc. Royal Soc. (London), A249, 367 (1959)]. Heat aging tests were performed by placing strips in a forced draft oven at various temperatures and obtaining the time when the samples flowed under their own weights to over double their initial lengths.

In general, the modulus of elasticity and ultimate strength are very significant in heat-recoverable materials. For the best use of such materials, they must be capable of a significant amount of stretching without splitting at elevated temperatures.

Various memory tests were performed on the heatrecoverable polyurethanes of this invention. These data are an indication of the capacity of the heat-recoverable material to return to its original shape upon heating. These tests were performed using a metal clamp device which served to stretch the polyurethane samples. The samples were dipped in a glycerine bath, either under tension or stretched in the bath and, while under strain, quenched in a water bath at ambient temperature. The polyurethane samples were 0.6 mm. thick strips about 4-5 mm. wide. The bench marks were generally 12.7 mm. apart. Final shrinking was accomplished on the expanded samples several hours and generally overnight after heating, stretching and quenching.

In the various examples which EXAMPLE 1 Various polyurethane compositions sold under the trade name Estane were mill mixed on a two-roll laboratory mill at a temperature of l50-l 75 C., and the resulting mixtures were pressed into 0.6mm. slabs. The slabs were then subjected to various irradiation doses, using a iMeV General Electric Resonant Transformer operating at 5-6ma. The modulus of elasticity and ultimate strength at 160 C. and gel content of these cross-linked polyurethanes were measured and the results are tabulated in ia'uie i As is shown in table 1, two difierent cross-linking monomers were incorporated in the thennoplastic polyurethane and irradiated to total does of 5,10,20, and 40 megrads. At 160 C., the polyurethane containing no cross-linking monomer requires much higher doses to obtain equivalent modulus than samples containing monomer. It can be seen in the data in table 1 that N,N-hexamethylene-bismaleimide and N,N'- methylene-bis-acrylamide are very effective cross-linking monomers.

EXAMPLE 11 Several of the cross-linked samples of example i were heat aged at temperatures of I75and 200 C. to determine the effect of cross-linked density on the heat aging properties. The results of these tabulated in table 2.

The data in table 2 shows that the cross-linked thermoplastic polyurethanes of this invention have vastly improved high temperature dimensional stability over the uncross-linked materials. The cross-linked polyurethane rubbers of this invention are, therefore, generally suitable over a wide range of temperatures.

EXAMPLE 111 20 stretching the samples while at temperature, and, while in the stretched condition, quenching in a cold water bath.

To test the heat recoverable characteristics of these materials, stretched samples were kept at room temperature for TABLE I Dose Monlomelr 5 Mrad, 160 C. Mrad, 160 C. Mrad, 160 C. Mrad, 160 C eve Monomer pts./l00 Percent Mod., US, Percent Mod., US, Percent Mod., US, Percent Mod., D S, Estane N 0. used pts./resln gel p.s.i. p.s.i. gel p.s.i. p.s.i. gel p.s.i. p.s.l. gel p.s.i. p.s.i. 5740XI Nothing 0 5. 0 5. 9 0 7. 4 13 25. 5 12 67 50. 0 23 5 l 0 18 69 46. 4 1!) 61 53. 5 30 113 51. 7 37 75 5740K! MBA 2 53. 1 41 129 56.5 75 160 54. 5 74 112 72. 4 123 122 4 58. 9 81 129 66. 2 82 156 72. i) 106 136 76. 3 161 113 68013 Nat Nothing 0 7 4 0 =10 23 32.3 8 52, a

95 52 7 52 55. 3 21 94 64. 0 32 58013 Nat MBA l 57. 3 13 88 59. 3 118 68.0 7 2 67. 6 23 94 71 5 60 127 76. 5 71 78. 1 4 70. 5 73 143 76. 9 116 149 77. 3 137 79. 1 58013 Net HMBMI 3 26. 0 29 29 33. 7 26. 5 63 47. 8 41 l 50. 6 36 67 55. 9 51 85 59. 5 49 2 67. 5 58 128 70. 0 79 112 70. 7 89 5740K? Nothing 0 0 2. 2 0 s 27 4a. 7 1a. 4 43 58.7 25 59 1 38. 4 15 51. 3 38 98 62. 3 44 94 68. 8 78 73 5740X2 MBA L... 2 43. 0 21 101 55.2 47 111 67. 5 75 72. 0 7 4 50. 2 39 104 64. 3 75 154 73. 4 109 76. 0 164 124 6740K? Nothing 0 15 1s 0 27 1s Trace 23 32 42. 7 3o 51 1 56.5 42 54 60. 4 40 60 65. 4 44 82 69.6 62 73 5740X7 MBA 2 56. 3 53 77 64.0 45 70 68. 8 58 63 73.0 80 7 4 68. 2 81 90 77.0 63 88 78. 4 97 82. 0 133 104 1 N ,N-methylene bis-acrylamide. 9 N ,N -hexamethy1ene bis-maleimlde.

TAB LE 2 Monomer levei Heat aging observations (pt./100 Dose, 0 Estane type pts.) mr. 200 C. C.

5" 1 1 None 0 Flowed in less than 1 hour Flowed in less than 1 hour, 5723221 do 20 do Slightly elongated at 1 hour,

otherwise OK atitter 72f;I hours. 40 d Elo ated Somea e15 ours, SHOXL do 0 ot l i erwlse OKatter 700 hours.

0 do Flowed in less than 1 hour. 20 do Do. 40 do Elongated after 1 hour, flowed after 5 hours. 5 Elonggted in less than 1 hour, flowed 4 ours. 20 Elongated some between 20-25 hours, OK after 700 hours.

otherwise OK after 87 hours. 40 Elongated some at 41 hours, other- Do.

wise 0K alter 87 hours. 1 20 Flower] in less than 1 hour Elongated after 5 hours, flowed at 13 ours. 58013 Nat MBA/2.... 40 do Melted at 13 hours. 5740X1 MBA 5 Elongated in 1 hour, otherwise OK after 36 hours. 5740X1 MBA/4.... 20 OK after 87 hours 1 OK after 700 hours. 5740X1 MBA/4.... 40 do Do. 58013 Nat" MBA/4.... 20 Elongated1 at 1 hour, flowed between Elqargaizad0 at 13 h mrs, otherwise OK 1 an 2 ours. a er ours.

58013 Nat MBA/4 40 Elongated at 1 hour, otherwise OK Elongated at 37 hours, otherwise OK after 87 hours. I after 700 hours. 1 5740X2 MBA /4 .v 40 Elohgat'e'd'at 18 hours, otherwise 0 K after 36 hours. 5740X7 None 5 Flawed in less than 1 hour 5740X7 MBA/2... 5 Elongated in less than 1 hour, otherwise O K after 36 hours. 5740X7 MBA/4.... 5 o

B eeame brittle somewhere between 87 and 231 hours.

and "porous" at 350-380 hours.

several days and subsequently placed in a glycerine bath at dif- 800"; l'lt innit Ultinnito RI tferent temperatures. The recovered dimension was then mea- 170 C. strength modulus strength tiiin sured. A comparison of uncross-linked and cross-linked WWW) polyurethane sold under the trade name Estane 5740Xl is 66 04 3,215 7. 355 407 shown in table 3. 5 69 36 333 TABLE 3 Elongation Final recovery Stretched (percent) (percent) increase elongation, after quenching in length over Expansion percent in water bath original length atbath (stretched at (1 hr. after quick temp, 0. bath temp.) release at 25 C.) 110 C. 160 G.

g?) 560 1% 2 176 1 3 4 pts. MBA/100 pts.

Estane 5740Xl at 80 160 100 5 Mrad 5 100 150 100 2 120 160 100 6 140 176 1% l2 160 200 l 10 Estane 5 740X1 100 200 mowed (not c1osslinked) 160 mowed As is shown in the foregoing data, the polyurethane materials of this invention are able to retain the elongation to a highdegree and yet upon the final recovery, by heating in a 160 glycerin bath, the original length is subsequently regained. As shown in the foregoing data, when the same experiment is performed on the thermoplastic polyurethane, which has not been cross-linked a low-degree of recovery is obtained at a temperature below the flow temperature of this polymer. it is not practicable to obtain a high-degree of recovery since the material flows at temperature necessary to obtain a highdegree of recovery.

EXAMPLE IV To demonstrate the chemical cross-linking of thermoplastic polyurethanes, two different polyurethanes sold under the trade name Estane were mill mixed at a temperature of 90-] 20bL C. with two different organic peroxides and with an without a cross-linking monomer. Following the mill mixing, slabs were cured in a mold at [70 C. for either 8or l0minutes, depending upon the specific peroxide used. The modulus of elasticity at 160 C. and percent gel were measured as described previously. Results are tabulated in table 4.

The 20 Mrad sample was then expanded to an external diameter of 0.250 inch to 0.255 inch. The technique and equipment used was that described in U.S. Pat. No. 3,086,242. The expanded tubing, when heated to 170 C., returned to substantially its original cross-linked dimension.

Samples of the tubing were placed in a forced draft oven at 200 C. The unirradiated material elongated within 5 minutes and flowed within 5 hours. The l0Mrad sample elongated 33percent within l5%hours and 60pcrcent within l20hours. The 20Mrad sample elongated lOpercent within lS'khours and 25percent within l20hours.

The use of the heat-recoverable polyurethane of this invention is simple and will be readily understood by those skilled in the art. in general, the polyurethane article is simply put into position for use and heat is applied to it, whereupon the article quickly recovers its original cross-linked configuration. For example, in the production of heat-recoverable rubber tubing, the thermoplastic polyurethane is first compounded and extruded into tubing form. The extruded tubing is then crosslinked. Thereafter, the tubing is subjected to stress while at elevated temperature. This stress may be applied, for example, by the application of pressure upon the inside of the tubing.

TABLE 4 Modulus of elasticity and ultimate Peroxide strength at 160 C. MBA level level Percent Estane type (pts./100 pts.) (1 pt./100 pts.) gel Mod. (p.s.i.) US (p.s.i.)

l N,N-methylene bis-acrylamide.

I Varox, 2, 5-dimethylhexane-2, fi-di-t-butyl diperoxide (the pure liquid), samples cured at In general, it can be seen that the peroxides cross-link these thermoplastic polyurethanes and higher elastic modulus values are obtained when cross-linking monomers are used.

EXAMPLE V One hundred pts. of the polyurethane, sold under the trade name Texin 480A were mixed with 4pts. N,N'-methylene-bisacrylamide at 180 C. on a hot, two-roll mill. The resultant mixture was extruded in a i kinch diameter plastic extruder using a rear zone temperature of 160 C., a center zone temperature of 190 C., and a head temperature of 190 C. The tubing had an average wall of 0.016 inch and an average internal diameter of 0.1 13 inch.

The tubing was then irradiated with high-energy electrons from a lMeV General Electric Resonant Transformer, using a beam current of 6ma. at lmegavolt. The tubing was irradiated to two different doses, l0and 20megarads.

The tubing had the following physical properties:

The quenching of the tubing while in the stressed state locks -the tubing into that state. The tubing therefore has a diameter greater than the original diameter of the extruded and crosslinked tubing, and it will remain in this condition indefinitely at normal storage temperature. The tubing is available for use at any time. For example, the expanded tubing may be placed over an article to be encased. Brief application of heat to the tubing will then cause it to shrink and attempt to return to its original cross-linked dimensions. This recovery permits the tubing to contract tightly around the article which had been inserted therein prior to the application of heat. Therefore, these material are particularly suitable for the encasement of cable wires and the like, as well as for many other purposes as will be apparent to those skilled in the art.

The thermoplastic polyurethane rubbers derive their unique properties from interchain hydrogen bonding (secondary cross-links). When these polymers are covalently cross-linked (primary cross-links) they are thus dually cross-linked. Other dual cross-linked elastomeric system exist which show the same unique heat recoverability over wide temperature ranges that the foregoing thermoplastic polyurethanes exhibit. An essential feature of these dual cross-linked elastomers is that they have two types of interchain cross-linking, primary crosslinking and secondary cross-linking. The concentration of the secondary cross-links is markedly decreased by the application of heat resulting in an elastic structure whose strength at elevated temperatures is dependent almost entirely upon the primary cross-links. It is envisioned that the primary crosslinks will generally always be heat stable covalent chemicals linkages, while secondary cross-links can be any interaction whose concentration or strength is subject to weakening upon the application of heat. Such secondary cross-linking of secondary chain interaction might be the result of ionic groups showing mutual attraction to a common ion, ionic groups of opposite charge, strong interchain forces exhibited by blocks of glassy'like polymer in an otherwise elastomeric material, and weak covalent linkages which are extremely unstable. These dually cross-linked elastomers exhibit elastomeric characteristics over a wide temperature range, and, in addition, exhibit the unique property of heat recoverability. Thus,

'elastomer articles of various shapes and sizes can be produced which have elastomeric properties over a wide temperature range and are heat recoverable in contrast to the conventional elastomers which exhibit no heat recoverability.

Having fully described the invention, it is intended that it be limited only by the lawful scope of the appended claims.

I claim:

1. A dimensionally heat unstable article capable of changing dimension upon the application of heat alone to assume a predetermined dimension and heat stable condition, said article made by a process comprising cross-linking a thermoplastic polyurethane article by irradiating it to a dose of at least about one megarad, deforming the article at a temperature within the range from about C. to l60 C., and quenching the article in the deformed state.

2. The article of claim 1 wherein quenching occurs at ambient temperature.

3. The article of claim 2 wherein said polyurethane article contains a cross-linking monomer prior to cross-linking.

4. The article of claim 3 wherein said polyurethane contains at least about 0.5parts by weight per IOOparts polyurethane resin of N,N'-methylene-bis-acrylamide.

5. The article of claim 3 wherein said polyurethane contains at least about 0.5 parts by weight per l00parts polyurethane resin of N,N '-hexamethylene bis-maleimide.

I i i t 

2. The article of claim 1 wherein quenching occurs at ambient temperature.
 3. The article of claim 2 wherein said polyurethane article contains a cross-linking monomer prior to cross-linking.
 4. The article of claim 3 wherein said polyurethane contains at least about 0.5 parts by weight per 100 parts polyurethane resin of N,N''-methylene-bis-acrylamide.
 5. The article of claim 3 wherein said polyurethane contains at least about 0.5 parts by weight per 100 parts polyurethane resin of N,N''-hexamethylene bis-maleimide. 